1. Field of the Invention
The present invention relates to a structure for use in contact with sea water or fresh water for a long time, and more particularly, to a structure capable of effectively preventing aquatic organisms for a long time from attaching thereto or an object covered therewith. The present invention also relates to a water-permeable structure for preventing the attachment of aquatic organisms to, for example, a sensor for use in water analysis, thereby ensuring prolonged optimum conditions for obtaining accurate water analysis. The present invention also relates to a filter excellent in preventing the attachment of aquatic organisms and allowing a long-term stable permeation of sea water or fresh water therethrough, and further relates to a cover for a bottom or a screw of ship easily attachable to or detachable from the bottom or the screw due to its good water permeability.
2. Description of the Related Arts
Examples of products for use under sea water or in partial contact with sea water for a long time include fishery equipment such as a fixed net for fishing and a fish preserve net for cultivating fish and shellfish, nautical equipment such as a floating nautical mark, a floating buoy and a mooring buoy, and civil engineering equipment such as a membrane and a fence for preventing water pollution. When these equipments are kept in contact with sea water for a long time, adhesive marine organisms attach to the surface thereof, and live and propagate there. Examples of such adhesive marine organisms include algae such as sea lettuce (ulva) and diatom, coelentera such as sea anemone (actinia), sponge such as beach sponge, annelids such as clam worm (nereid), tentaculatae such as sea moss (bryozoan), mollusk such as moule (Hiatula diphos), arthropod such as barnacle, and protochordate such as sea squirt (ascidian). The attachment of marine organisms to the above equipments causes a problem of preventing the equipment from exhibiting the expected function sufficiently.
Recently, studies has been made on the technique in which the properties of river water and sea water are continuously monitored by measuring various items such as dissolved oxygen content, pH, temperature, salt content, ammonia content and turbidity, and the obtained data are analyzed to predict red tide, rapid environmental change and occurrence of natural hazard so as to use the results in cultivating fish and preventing natural hazards. Sensors and systems to obtain automatically these data have been developed and are under examination for practical use.
However, when a sensor is continuously kept in water to obtain water data, aquatic organisms such as those described above are attached to the detector of the sensor. As a result thereof, within a period as short as a few days, the sensor becomes out of order to give abnormal data and the accurate measurement becomes impossible. Therefore, it is necessary that the aquatic organisms be frequently removed or the sensor be repaired or replaced with new one. Since the monitoring system is frequently placed far from the shore, the repair or the replacement is labor-intensive, thereby retarding the wide application of the automatic monitoring system.
To prevent aquatic organisms from attaching to a structure kept in contact with sea water for a long time, the following method has been employed heretofore.
The conventional method generally employed is to treat a structure with an organotin compound such as tributyltin oxide, triphenyltin hydroxide, triphenyltin acetate and triphenyltin chloride. However, the organotin compound generates awfully unpleasant or irritating smell during the treating operation. Moreover, it has recently become clear that the organotin compound is accumulated in fish body to cause deformation and death of fish, and that eating the contaminated fish is detrimental to the human health. Therefore, the use of the organotin compound is voluntarily restricted in the fishery field and may be completely prohibited before long.
Therefore, it is desired to develop a new method as a substitute for the method using the organotin compound having the above severe drawbacks.
In one of such new methods, the equipment is treated with an organic sulfur-nitrogen compound such as urea compound, benzimidazole compound, benzothiazole compound, thiophthalimide compound and sulfonylpyridine compound. This method is an attempt to apply the organic sulfur-nitrogen compound to preventing the attachment of aquatic organisms based on the fact that the organic sulfur-nitrogen compound has been widely used as an agricultural chemical, a bactericide and a fungicide. It has been known that the organic sulfur-nitrogen compound is less toxic to human body and fish, and decomposes into a non-toxic compound after accomplishing its function of preventing the attachment of aquatic organisms.
As a method of using such a highly safe organic sulfur-nitrogen compound having a high ability of preventing the attachment of aquatic organisms, that is antifouling effect, the following method has been proposed.
In this method, for example, a coating material comprising a mixture of the organic sulfur-nitrogen compound and an oily resinous binder such as a drying oil exemplified by linseed oil, tung oil, soy bean oil, dehydrated castor oil, safflower oil and fish oil; a phenol resin; an oily resinous varnish; and an alkyd resin which is a reaction product of a polyhydric alcohol and a dicarboxylic acid is applied to the surface of the equipment and then cured. Although this method shows the desired effect in early stage after immersed into sea water, the effect of preventing the attachment of aquatic organisms disappears in a short period of time because the effective component and the resin component in the coating are leached into sea water or the coating is detached from the coated surface by wear due to its poor bonding strength.
As mentioned above, by using the organic sulfur-nitrogen compound, no method having a high and sufficiently durable effect on preventing the attachment of aquatic organisms is obtained in any of the fishery field, nautical field and civil engineering field.
The above problems are also found in sensors for water analysis and filters used in sea water and fresh water. To solve the problems, it has been attempted to use a sensor having a cover coated with an antifouling coating containing the organotin compound. However, this method was found not suitable for practical use due to environmental pollution, toxicity to human body and marine organisms and adverse effect on the water analysis. Alternatively, when the detector of the sensor is directly coated with the antifouling coating, the detector fails to become direct contact with water, thereby making the water analysis impossible.